For that fish to live for 40+ years was nothing short of a miracle. Key components in any monster fishes longevity are tank size, water quality and diet. Look after all three and you have a chance of rearing a gnarly old timer.
This guy, I can only guess, must have kept that fishes water pristeen, because he certainly fell way short on the other two requirements, tank size and diet.
If he didn't keep his water perfectly clean and he was failing the fish on all three fronts then "miracle" isn't the word I would choose to describe it's old age.
In fact, I can't even think of a word!!! It just defies the odds of everything we all know about keeping healthy fish.